The present invention relates to a spindle motor for hard disk drives having a hydrodynamic bearing arrangement.
Spindle motors for hard disk drives essentially consist of a stator, a rotor and at least one bearing system arranged there-between. The electromagnetically driven rotor is rotatably supported in relation to the stator by means of a bearing system. Roller bearings and hydrodynamic journal bearings can be utilized in such bearing systems.
It has been established recently that use of hydrodynamic bearings in such spindle motors is more advantageous than use of roller or ball bearings. Specific advantages of hydrodynamic journal bearings compared to roller bearings lie in their greater running precision, their insensitivity to shock and the need for fewer components. Since the sliding parts do not touch each other at the nominal speed, they have low wear and tear and operate almost noiselessly.
Such a hydrodynamic bearing arrangement is known, for example, from U.S. Pat. No. 5,847,479. In this bearing system, a fluid is used as lubricant which is located in a bearing gap that separates the two bearing surfaces of the parts moving in relation to each other. In operation, the lubricant, preferably oil, forms a load-bearing lubricating film between the bearing surfaces.
The known bearing arrangement includes a bearing sleeve and a shaft which is arranged in an axial bore of the bearing sleeve. The shaft rotates freely in the bearing sleeve and forms a radial bearing together with the sleeve. A thrust plate, which rotates in a corresponding recess in the sleeve, is positioned approximately in the middle of the shaft and, together with a cover plate, forms a thrust bearing. The thrust bearing prevents any displacement between the shaft and the sleeve along the rotational axis. At least one of the mutually interacting bearing surfaces of shaft and sleeve, and thrust plate and cover plate has a groove pattern which, due to the relative movement caused by rotation, produces a pumping action in the lubricant, thus creating the hydrodynamic pressure necessary for bearing stiffness.
In the bearing arrangement described above, the thrust bearing plate is located approximately in the middle of the shaft. The thrust bearing or bearings are thus positioned between the radial bearings along the rotational axis.